Reverse left septal activation during intermittent left bundle-branch block: indirect proof for concealed retrograde left bundle-branch activation.

Concealed retrograde activation of bundle branch has been often proposed for explaining the persistence of functional rate-dependent bundle-branch block, but direct proof of such a mechanism in man has rarely been demonstrated. We report intracardiac recordings compatible with a reversal of activation of the left basal interventricular septum during intermittent left bundle-branch block. In our case, however, retrograde activation into the left bundle-branch cannot be recordable, probably due to the concealment within the muscular septal activation, even if this may explain the perpetuation of bundle-branch block according to the linking phenomenon.